The present invention relates to a ball, more particularly for sporting games, which consists of thermoplastic material, and to the method of manufacturing same.
It is known that the balls used in playing sports games and notably competition games must meet certain requirements as to weight, size, rebound, resilience, resistance to high and low temperatures, etc . . .
The increasing popularity of sports has led to the development of synthetic raw materials for manufacturing balls at a cost lower than that of leather balls, since leather, the hitherto essential and standard material utilized in the manufacture of sports balls, is rather expensive, while endeavoring to impart to the plastic balls properties as close as possible to those of leather balls.
A first method of manufacturing sports balls consisted in operating by rotational molding under standard conditions from thermoplastic materials in the initial form of dough or in crosslinked form, or from thermoplastic materials adapted to be cross linked during the manufacturing process.
More recently, vinyl chloride products have also been used either in the form of dough or plastisol or in the form of dry powder.
Additives intended for modifying more particularly the ball resilience in order to obtain a predetermined rebound have also been incorporated in these synthetic materials.
Thus, the results obtained up to now led to the manufacture and sale of thermoplastic balls, but all attempts to obtain an exact replica of a leather ball did not give fully satisfactory results.
When selecting new raw materials having the desired properties it is also necessary to take due account of the temperatures at which the game is to be played with the ball. Thus, suitable limits to this range may be -15.degree. C. to +30.degree. C., which corresponds to temperatures in temperate areas during the seasons in which the game can be played. Besides, the practice of football and like contests are excluded outside this temperature range. It is clear that a product tending to harden considerably at a temperature below 0.degree. C. is not suitable.
Moreover, the rebound obtained with a competition ball (U.S. football, Association football, rugby union, basket ball, volley ball) should be set for the above-mentioned temperature range in which the game can be played; in fact, leather is basically insensitive to temperature. Thus, by way of example, a football association ball must rebound at 65 percent of the initial height.
Since the ball must be utilized in relatively variable seasons, it must also have a good resistance to humidity. Furthermore, it must be distortion-free, especially in the case of a football association ball exposed to very severe strain (percussion, etc.).
The external layer must have a good resistance to abrasion, since the ball might be used on a sandy pitch or indoor, especially in the case of handball, volley-ball and like games.
In actual practice, all these properties are not required in the case of a play ball for children; however, the possibilities afforded by selecting the proper materials will obviously lead to a very substantial improvement in the properties of play balls of this type.